The ecology of fishes and mosquitoes of the lower Gambia River floodplains

Abstract

This study investigated aspects of the ecology of fishes and mosquitoes using floodplains in the lower reaches of the Gambia River, a semi-arid river system. The Gambia River, situated at the edge of the Sahel, represents one of the few remaining major river systems not to have been impacted by any impoundments. It was hypothesised that seasonal variations in physical variables particularly related to the pattern of flooding and drying influence fish species distribution; the presence of some of these species probably also influencing mosquito larvae distribution. Fish and mosquito larvae were sampled along two lateral transects on the floodplain in The Gambia, from May to November 2005 - 2007. Water conductivity, pH and water depth all influenced fish species richness and bolongs (creeks) held greater species richness compared with other floodplain habitats, probably because they acted as conduits for fish moving on and off the floodplain. Species richness and catch biomass increased rapidly following the first rains and then declined. The Guinean tilapia, Tilapia guineensis, was the dominant species on the floodplains and its catches were positively associated with higher levels of conductivity and dissolved oxygen, shallower water and less vegetation cover. The Guinean tilapia is primarily an iliophage and the catches varied seasonally. Controlled experiments using T. guineensis in tanks with shallow water barriers showed that fish density plays a significant role in triggering fish emigration, whereas a lack of food available caused an increase in exploratory behaviour but with no impact on successful emigration. The influence of fish on the distribution of mosquito larvae on the floodplains was investigated. Semi-field trials were used to test two possible mechanisms for this influence: direct predation and oviposition avoidance due to the presence of fish chemical cues. The presence of T. guineensis decreased the chance of finding culicine larvae in the field possibly due to culicine mosquito avoiding ovipositing in habitats with fish present as fewer culicines oviposited in experimental tanks with fish, suggesting that ovipositing culicine females avoid water with fish. In contrast, oviposition by anophelines was unaffected by fish. Both fish species tested, T. guineensis and a common insectivore, Epiplatys spilargyreius were effective predators removing all late-stage culicine and anopheline larvae. In order to determine the relationships between mosquito distribution and the physical environment in an urban setting, mosquitoes were collected and environmental physical parameters were measured in and around Farafenni town, located at the edge of the foodplains in The Gambia. Levels of reactive phosphorus, distance from the nearest house, turbidity, amount of vegetation cover and the presence of algae all influenced the occurence of mosquito larvae collected in and around Farafeimi town. Anopheles arabiensis was found closer to houses than Anopheles gambiae sensu stricto and the presence of algae was a strong indicator of the presence of anopheline larvae in general. Sites dominated by anophelines had higher turbidity levels compared to sites in which culicines dominated. Production of pupae was associated with high levels of reactive phosphorus concentrations. The results are discussed in the context of world climate change, anthropogenic impacts on large rivers and increased urbanisation.